Many wireless communication systems assign transmission resources using one or more schedulers associated with a base station, which serves multiple mobile terminals. Typically, these resources have a time division multiple access (TDMA) component, wherein communications between the base station and a select mobile terminal are assigned to a given time slot. For downlink communications wherein the base station transmits data to the mobile terminal, the base station's downlink scheduler receives data for transmission to the mobile terminal, and allocates a certain time slot in which to transmit data to the mobile terminal. Notably, transmissions to different mobile terminals are assigned to different time slots to facilitate an ordered transmission of data amongst the mobile terminals. Similarly, in uplink communications wherein the mobile terminal transmits data to the base station for delivery across the network, the base station's uplink scheduler determines when the mobile terminal can transmit information to the base station, and via control signaling, instructs the mobile terminal of the time slots in which to transmit data to the base station.
Typically, packets of data are sequentially transmitted between the base station and the mobile terminal in the form of frames, which may be lost or corrupted due to channel conditions during transmission. The major objectives of layer 2 protocols in wireless access networks are to perform resource sharing among multiple services and multiple users and to provide improved radio link quality and reliability by implementing a retransmission mechanism for lost or corrupted frames associated with non-delay-sensitive services and applications. For CDMA standard, the radio link protocol (RLP) uses an automatic repeat request (ARQ) protocol to monitor incoming frames and request retransmission of lost or corrupted frames. An Internet Protocol (IP) based RLP design allows an RLP frame to encapsulate an IP packet or fragment of an IP packet. Each RLP frame header includes a sequence number to maintain the integrity of RLP frames flowing over the wireless link. In a negative acknowledgment (NAK) based RLP ARQ scheme, after identifying the loss of an RLP frame at the receiver RLP entity, a NAK is sent to the transmitter RLP entity. The NAK identifies the lost RLP frame and triggers retransmission of the lost RLP frame by the transmitter RLP entity. Lost RLP frames are determined by checking the sequence numbers of subsequently received RLP frames. Once an RLP frame is lost, a significant amount of time may pass before receiving a subsequent RLP frame, which is capable of providing information to determine that the previous RLP frame was lost.
For example, the reception of RLP frames N and N+2 in a row indicates that RLP frame N+1 was lost. After receiving RLP frame N+2, the receiver RLP entity sends a NAK request indicating a frame was lost. In a high-speed wireless Internet access system, arrival times for frames often vary greatly due to the high non-stream-like nature, or burstiness, of packet applications. If frame N+2 arrives at the receiver a relatively long time after RLP frame N, then the receiver RLP layer will take a longer period of time to identify the possible loss of RLP frame N+1. The result is a longer wireless link delay for RLP frame N+1.
In a traditional RLP scheme, retransmissions are under control of the transmission RLP entity. That is, the receiver RLP entity is only responsible for informing the transmission RLP entity that a given RLP frame was lost or received in error. The transmission RLP entity is responsible for determining when to retransmit that RLP frame and how many copies to include based on requisite retransmission parameters. The retransmission parameters are primarily a function of the QoS level associated with the data stream being considered. For example, the retransmission parameters may relate to acceptable error rates and transmission delays.
As noted above, the base station controls resources for uplink communications. If the mobile terminal does not have any extra transmission resources to use for retransmission purposes, then the mobile terminal's RLP entity would need to request additional transmission resources from the base station to perform any required retransmissions. When an RLP frame is lost, the base station's RLP entity sends a NAK to the mobile terminal's RLP entity. The mobile terminal has to determine what additional transmission resources are required for retransmission and then request those resources from the base station via a transmission request. Under control of the uplink scheduler, the base station must grant the additional resources for the mobile terminal, schedule transmission of the transmission grant, and transmit the transmission grant to the mobile terminal. Upon receipt of the transmission grant, the mobile terminal's RLP entity is then finally able to initiate retransmission of the frame and any copies thereof. The process involves an extra round of signaling between the mobile terminal and the base station, thereby increasing both the delay due to retransmission and the amount of signaling overhead carried on both uplink and downlink channels.
Accordingly, the traditional RLP schemes incur additional delays and signaling overhead due to the need for the mobile terminal's RLP entity to request additional transmission resources from the base station's uplink scheduler for each retransmission. The additional signaling delays reduce the QoS levels for uplink communications. Further, the extra signaling overhead will likely have a negative impact on total system capacity, since it will reduce the amount of user data that can be transmitted over both the uplink and downlink. As such, there is a need to minimize the time required to identify lost or corrupt RLP frames and to decrease the delay in retransmitting the lost or corrupt RLP frames without sacrificing reliability.